Throughout the specification, the terms “source” and “transmitter” are used interchangeably, as are the terms “receiver,” “sensor” and “detector”.
Use of marine controlled source electromagnetic (mCSEM) surveying, also referred to as sea bed logging (SBL), for mapping hydrocarbons over shallow prospects in deep water is disclosed in patent specification number GB 01/00419. Further examples of this technique are disclosed by Eidesmo T, S Ellingsurd, L M MacGregor, S Constable, M C Sinha, S Johansen, F N Kong, and H Westerdahl 2002 Sea Bed logging (SBL), “A new method for remote and direct identification of hydrocarbon filled layers in deepwater areas”: First Break 20 144-152, and Ellingsrud S, T Eidesmo, S Johansen, M C Sinha, L M MacGregor, and S Constable 2002 “Remote sensing of hydrocarbon layers by sea bed logging (SBL); Results from a cruise offshore Angola”: The Leading Edge 21 972-982.
GB 2 390 904 discloses an electromagnetic surveying technique using a vertical electric dipole and a vertical magnetic dipole or naturally occurring electromagnetic fields.
Another known type of electromagnetic surveying technique is known as the multi-transient electromagnetic (MTEM) method and an example of this is disclosed in U.S. Pat. No. 6,914,433.
The mCSEM/SBL technique is based on the fact that hydrocarbons in a subsurface are significantly more resistive to electromagnetic waves than non-hydrocarbon-bearing layers, such as shale or sandstone containing saltwater. The resistivity of shale is in the range of 0.5-3 Ωm and that of water-filled sandstone is around 1 Ωm, whereas that of hydrocarbon-filled sandstone is in the range 5-200 Ωm. Hydrocarbons may therefore be detected by transmitting electromagnetic fields into the subsurface and recording the returning signal at a range of distances or “offsets” from a source. Such an electromagnetic surveying technique is sensitive to the types of fluid in the rock.
A mCSEM/SBL survey typically emits electromagnetic signals close to the sea bed from a powerful electric source. This is generally a horizontal electric dipole (HED) transmitter driven by a low frequency alternating current (AC) of quasi-square waveform. The first several harmonics may be processed so as to increase the bandwidth.
In a typical mCSEM/SBL survey, one or more receivers are deployed along a line on the sea bed or across a section of the sea bed using high-symmetry or irregular receiver grids. Each of the receivers includes an instrument package, a detector, a flotation device and a ballast weight. The detector may comprise, for example, a three component electric dipole detector set and a three component magnetic dipolar detector set. In each of these sets, two dipole detectors are oriented in the horizontal plane as an orthogonal pair and the third dipole detector is oriented in the vertical direction.
Once the survey has been completed and the data collected, the receivers may be retrieved. A telemetric signal from the survey ship prompts each receiver to detach itself from its ballast weight and float to the surface by means of the flotation device, which typically comprises a top-mounted buoyancy system. The receiver position is monitored by a hydro acoustic tracking system. Once the raw data have been downloaded to a computer, they are collated and processed for subsequent data interpretation. The end product is typically an electromagnetic map of the surveyed area, in which hydrocarbon-bearing layers can be distinguished from other layers.
The three major pathways for propagation of an mCSEM/SBL signal are through the sea water, through the subsurface, and through the air. The direct field through the sea water is the signal which is transmitted directly from, for example, an electric dipole source to a receiver. This field dominates in amplitude at short source-receiver separations or offsets but is strongly damped at larger offsets due to a combination of geometrical spreading associated with the source dipole geometry and skin-effect-related exponential attenuation.
The signal that travels partly through air is called the source-induced “airwave”. The airwave is dominated by the signal component that diffuses upwards from the source to the sea surface, then propagates through the air at the speed of light with no attenuation, before diffusing back down through the seawater column to the sea bottom where it is picked up by the receivers.
The subsurface structures are, in general, more resistive than the sea water. As a result, skin depths in the subsurface are larger than those in sea water so that the electromagnetic fields propagating in the subsurface before returning to the seabed at intermediate to long offsets are less attenuated than the direct field.
A hydrocarbon-filled reservoir has relatively high resistivity compared with shales and water-filled sandstones of the subsurface. The field of main interest for hydrocarbon mapping is related to the energy propagating downwards from the source into the subsurface and then interacting with the resistive reservoir before returning upwards at intermediate to large offsets. Thus, the electric fields at the receivers should be larger in magnitude over resistive subsurface structures such as hydrocarbon reservoirs than the more-attenuated background electromagnetic fields caused by host sediments. This is related to the lower attenuation experienced by the component of the electromagnetic signal that travels along the higher resistivity hydrocarbon filled reservoir. Thus, when an electromagnetic field propagates over a long distance in hydrocarbon reservoirs, the amplitude of the detected signals dominate those signals which have propagated in the water-bearing sediments. This “enhancement” in electric field amplitude at long source-receiver separations (compared to the depth of the reservoir) allows hydrocarbon reservoirs to be detected.
It is known, however, that an increase in electromagnetic field amplitude need not solely be related to the presence of hydrocarbons. Also, local large-scale resistive bodies other than hydrocarbon reservoirs beneath the seabed can significantly affect the electromagnetic fields due to longer skin depths with increasing resistivity. Increasing resistivity structures with depth are a feature of some submarine sedimentary basins and are known to arise due to the progressive explusion of conductive pore fluids with increasing depth by rising overburden pressure. Accordingly, in order to determine reliably whether an enhancement in electric field amplitude is caused by a subsurface hydrocarbon reservoir or whether it is caused by local large-scale resistive structures, independent information about the large background structures in the survey area is needed.